"API viscosity ratings for gear oils are not directly comparable with those for motor oil, and they are thinner than the figures suggest. For example, many modern gearboxes use a 75W90 gear oil, which is actually of equivalent viscosity to a 10W40 motor oil."

Several instances around this forum, people use a specifically designed 5 or 10W40 oil from Europe in the getrag for racing. Since mines a DD, I use the ford honey.

When I worked in Germany, 10W-40 THERE was the same viscosity as 10W-40 in North America. Like I said, I have no idea where you get that information. cSt measurements are not 'custom made' to any fluid, they are a standardized measurement...

The Amsoil data support exactly what my point is. Motor oil is not even close to GL-4 lubricant. I think there are maybe three or four of us on this entire forum who can interpret what the spec sheets tell us. I don't mean anything as a personal attack but you're posting information that contradicts your hypothesis.